"Saturday will be critical," Mr De Mistura said. "No doubt, there will be no shortage of attempts to undermine this process."

Earlier, Russian warplanes carried out intense air strikes on rebel strongholds in Syria hours before the ceasefire was due to come into force, the Syrian Observatory for Human Rights monitor said.

Russia dismissed the reports and said it was continuing strikes on "terrorist organisations" ahead of the truce.

The partial ceasefire between regime forces and non-jihadist rebel fighters took effect at 10pm Irish time.

It does not include the so-called Islamic State jihadist group and the Al-Qaeda affiliated Al-Nusra Front.

"From last night to this morning there have been Russian air strikes that are more intense than usual on rebel bastions including on Eastern Ghouta east of Damascus, in the north of Homs province and in the west of Aleppo province," Observatory head Rami Abdel Rahman said.

US President Barack Obama warned Russia and Syria that the "world will be watching" when the ceasefire goes into force.

"Even under the best of circumstances, we don't expect the violence to end immediately," Mr Obama said.

Air strikes in the non-jihadist rebel-held Qabtan al-Jabal area of Aleppo province killed eight members of the same family, including three children, on Thursday night, the monitor said.

"There were at least 25 air strikes on Eastern Ghouta," a main rebel bastion where the predominant opposition faction is the Jaish al-Islam movement, Abdel Rahman said.

Russian spokesman Dmitry Peskov dismissed the Observatory's reports.

The Russian air force is certainly continuing its operation in Syria but against "terrorist organisations," he said.

Since Moscow launched air strikes in Syria in September, it has been accused of hitting non-jihadist rebel groups in support of the Damascus regime, a longtime ally.

Abdel Rahman said the implementation of the partial truce will be complicated as Nusra are allied with rebels - Islamist and non-Islamist - in several parts of Syria.

Their "territories overlap especially in Idlib and Aleppo provinces. It will be very complex," Abdel Rahman said.

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